I was just watching Top Gun and paused to ask this burning question.
In the opening scenes of the movie they are showing jets taking off from an aircraft carrier and behind the jets they have a wall that raises up behind the jet engines.
Why is that wall there? Does it increase the forward thrust of the jet? Are they trying to protect whatever is behind the wall?
Side Question: Does having a wall to “push against” increase the thrust off a rocket or jet engine?
To me it does not seem like it would. In my mind a rocket is a box with a hole in it. Inside the box there is an explosion and since the force of the explosion can only push on the walls of the box and not the hole the box moves in the direction opposite the hole. A wall on the outside of the hole not attached to the box itself does not seem to me like it would have any effect on the box.
It’s a blast screen. It’s there to keep the jet blast from injuring the deck crew or causing damage to equipment. If you have ever been behind a jet aircraft you’ll get the purpose of the blast screen pretty quick. As kids we used to stand on a wall at the end of the Fort Lauderdale airport’s main runway. We were about a hundred yards from the jets as they throttled up for take off. Even at that distance, the exhaust would blow us of the wall. We would see who could stay on the wall the longest. Great fun!
Basically, no. Thrust is based on momentum transfer: e.g. for a rocket, particles accelerated downward result in upward momentum, which is not altered by what happens to the particle after it leaves the rocket.
A minor exception would be if the wall causes the particle to rebound and strike the back of the rocket - this would add a small amount of additional thrust.
People still do this on St. Maarten, where the end of a runway is separated from a small public beach by just a service road. sometimes it doesn’t turn out so well.
Commercial jet engines develop a lot of thrust (~60,000 pounds each for the latest version of the 747), but as they are ducted-fan engines, they do it by moving a lot of air at modest speeds (here “modest” still means hundreds of MPH). In contrast, The two engines on an F-18 deliver a total of 35,000 pounds of thrust in full afterburner mode (for danger purposes the two exhaust streams may be considered as one since they are immediately adjacent to each other), and as they are not ducted-fan engines, their thrust comes from a relatively small quantity of exhaust gas being fired out the back of the engine at very high speed. On St. Maarten, crazy beachgoers are separated from the aircraft by about 250 feet; on an aircraft carrier, things (and deck personnel) can be considerably closer to the back end of departing aircraft, thus the need for the blast deflectors.
Yes. The thrust-to-weight ratio is anywhere between 0.27 and 0.56, depending on how much meat, luggage and fuel you stuff on board. Tires typically have a coefficient of friction somewhere close to 1, and even if the brakes may have a power absorption limit (which dictates decel rate when at high speed), they will have enough clamping force to keep the wheels from rolling at full thrust.
Either you’re not standing on the brakes hard enough, or your brakes need servicing.
Fix your brakes.
Seriously one of the standard diagnostic tests when diagnosing an auto trans problem is called a stall test.
Put the car in drive, stand on the brakes and floor the accelerator.
Since there is not a huge gaping hole in the back wall of every transmission shop it can be assumed that the brakes are in fact more powerful than the engine.
-they were operating at something close to max thrust with just the parking brake engaged, which only applies braking effort to some of the wheels. Once the aircraft started moving, they disengaged the parking brake and applied the brake pedals. That would have activated braking on all wheels - except that the crew simultaneously provided a large nosewheel steering input, causing the aircraft to limit the available braking effort. Had they kept the nosewheel pointing straight forward, they aircraft would have allowed them to access maximum braking effort, and the plane would have stopped nicely.
